The differentiation of mouse thymocytes into mature T lymphocytes will be dissected into sequences of discrete molecular events. The expression of four developmentally regulated polypeptides will be studied in isolated subpopulations of thymocytes which are in different stages of maturation. Each cell population will be characterized according to the rate of translation, post-translational processing reactions, and mRNA pool size it shows for each of the gene products. Purified populations of immature thymocytes will then be cultured under conditions which allow them to differentiate in vitro. At intervals throughout the incubation, the expression of each of the four gene products in the cultured cells will be monitored by biosynthetic pulse-labeling, cell surface radiolabelling, and in vitro translation of thymocyte RNA, to identify points at which the pattern of expression changes. The sequence of biochemical changes defined by these studies will be correlated with the development of glucocorticoid resistance and mitogen responsiveness in the thymocytes as they mature.